When I receive calls on my A20, including those to Google Voice, the phone vibrates along with the ringtone...even though "Vibrate while ringing" is switched off in Sounds and Vibration settings.
Also, vibration is switched off in the Google Voice settings. Why is it still happening??
A cheat while you are waiting a perfect answer could be ajust the intensity of vibration for calls to none. I don't know if your A20 has same option that my phone, but I tried for me and woks